Field of the Invention
The present invention relates to a dynamic pressure bearing, a spindle motor using the dynamic pressure bearing, and a rotary device having the spindle motor as a driving source and, more specifically, to a spindle motor in which a rotor is supported by a stator through the dynamic pressure bearing.
Dynamic pressure bearings are suitable as bearings of a rotary device in the fields of computers and copy machines because they can be miniaturized and can operate with smooth, high speed revolution. Particularly, as an air dynamic pressure bearing does not use a lubricant such as oil, a rotator such as a magnetic disk is never soiled. As a result, the air dynamic pressure bearing is adopted widely for a rotator device. However, the air dynamic pressure bearing has disadvantages, including that stiffness of the bearing is very low and that production is difficult because it requires a bearing gap of several micron order. In view of the foregoing disadvantages, a liquid dynamic pressure bearing having high stiffness and which is easy to produce has been developed.
The prior art is disclosed in U.S. Pat. No. 5,487,608. The device is a shaft rotation type spindle motor having a liquid dynamic pressure bearing in which lower end of the bearing is closed end, upper end of the bearing is opened end, and a radial bearing portion is formed at upper portion and a thrust bearing portion is formed at lower portion. When the spindle motor rotates at high speed, the radial bearing portion and the thrust bearing portion are constructed so that net flow of lubricating oil by dynamic pressure orients to the closed end direction at the radial bearing portion, net flow of lubricating oil by dynamic pressure orients to radial bearing portion direction, and net flow connected with these flows orients to the closed end of the bearing. Therefore, lubricating oil does not leak from the bearing when the motor rotates at high speed. A capillary seal is provided at the opened end so that lubricating oil does not leak out of the bearing when the motor stops.
A prior art bearing assembly is disclosed in U.S. Pat. No. 5,487,608 which is incorporated herein by reference. FIG. 4 of U.S. Pat. No. 5,487,608 has been replicated herein as FIG. 9. The bearing assembly comprises a shaft rotation-type spindle motor having a liquid dynamic pressure bearing. The bearing assembly comprises a stationary sleeve 70 has a lower portion 40, an interior surface forming a bushing 72, a tapered surface 162, a vertical surface 181, and an outer circumferential surface 182. A rotating shaft 52 constituting a cylindrical radial bearing member is mounted for rotation relative to the stationary sleeve 70. The rotating shaft 52 has hubs 174 and 180 and defines a radial bearing portion. Upper and lower journal bearings 130, 132 are disposed between the stationary sleeve 70 and the rotating shaft 52. A thrust plate 74 and a counterplate 76 define a thrust bearing portion of the bearing assembly. The thrust plate 74 has bearing surfaces 120, 122 and is stepped at 75 into a lower end of the rotating shaft 52 and extends into a recess 90 defined by a lower end of the bushing 72 and an upper surface of the counterplate 76. The counterplate 76 is pressed against the lower portion of the stationary sleeve 70 and sits in against a step as shown at 78. Immediately below the counterplate 76 is located a shield 84 which is provided to close the bearing assembly from the outside. The bearing assembly is further provided with gaps 92, 94, 96, recesses 80, 90, a bores 102, 134, a groove 104, a reservoir 100, equipressure grooves 136, 150, a capillary seal 160, a gas trap 170, and a labyrinth seal 172. A lower end of the bearing assembly is closed, an upper end of the bearing assembly is opened, the radial bearing portion is formed at the upper end, and the thrust bearing portion is formed at the lower end.
When a spindle motor of the bearing assembly rotates at a high speed, the radial bearing portion and the thrust bearing portion are constructed so that the net flow of lubricating oil by dynamic pressure orients in the closed end direction at the radial bearing portion, the net flow of lubricating oil by dynamic pressure orients in the radial bearing portion direction, and the net flow connected with these flows orients to the closed end of the bearing. Therefore, lubricating oil does not leak from the bearing when the motor rotates at high speed. The capillary seal 160 is provided at the opened end so that lubricating oil does not leak out of the bearing when the rotor stops.
The spindle motor disclosed in U.S. Pat. No. 5,487,608 is a useful liquid dynamic pressure bearing which has high stiffness and which is easy to produce as compared to an air dynamic pressure bearing. However, construction of the liquid dynamic pressure bearing is complex. As shown in FIG. 9, two members, namely the thrust plate 74 and the counterplate 76 are needed to form the thrust bearing portion. A dynamic pressure generating groove which does not function properly due to its construction must be formed at the radial bearing portion and the thrust bearing portion. Furthermore, the bores 102, 134, a radial dynamic pressure generating portion, and a thrust dynamic pressure generating portion must be formed respectively at the rotating shaft 52. As a result, it is difficult to miniaturize and manufacture the liquid dynamic pressure bearing having the foregoing construction.